Re: [PATCH wq/for-3.14-fixes] workqueue: ensure @task is valid across kthread_stop()

[Lai Jiangshan <laijs@cn.fujitsu.com>]

On 02/19/2014 05:37 AM, Tejun Heo wrote:
> Hello, Lai.
> 
> I massaged the patch a bit and applied it to wq/for-3.14-fixes.
> 
> Thanks.
> -------- 8< --------
>>From 5bdfff96c69a4d5ab9c49e60abf9e070ecd2acbb Mon Sep 17 00:00:00 2001
> From: Lai Jiangshan <laijs@cn.fujitsu.com>
> Date: Sat, 15 Feb 2014 22:02:28 +0800
> 
> When a kworker should die, the kworkre is notified through WORKER_DIE
> flag instead of kthread_should_stop().  This, IIRC, is primarily to
> keep the test synchronized inside worker_pool lock.  WORKER_DIE is
> first set while holding pool->lock, the lock is dropped and
> kthread_stop() is called.
> 
> Unfortunately, this means that there's a slight chance that the target
> kworker may see WORKER_DIE before kthread_stop() finishes and exits
> and frees the target task before or during kthread_stop().
> 
> Fix it by pinning the target task before setting WORKER_DIE and
> putting it after kthread_stop() is done.
> 
> tj: Improved patch description and comment.  Moved pinning above
>     WORKER_DIE for better signify what it's protecting.
> 
> CC: stable@vger.kernel.org

I think no one hit this bug. So I add this stable TAG?

(Jason's bug-report drives me to review the workqueue harder,
and I found this possible bug, but I think it is irrespective
with Jason's bug-report.)

> Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
> Signed-off-by: Tejun Heo <tj@kernel.org>
> ---
>  kernel/workqueue.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> index 82ef9f3..193e977 100644
> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -1851,6 +1851,12 @@ static void destroy_worker(struct worker *worker)
>  	if (worker->flags & WORKER_IDLE)
>  		pool->nr_idle--;
>  
> +	/*
> +	 * Once WORKER_DIE is set, the kworker may destroy itself at any
> +	 * point.  Pin to ensure the task stays until we're done with it.
> +	 */
> +	get_task_struct(worker->task);
> +
>  	list_del_init(&worker->entry);
>  	worker->flags |= WORKER_DIE;
>  
> @@ -1859,6 +1865,7 @@ static void destroy_worker(struct worker *worker)
>  	spin_unlock_irq(&pool->lock);
>  
>  	kthread_stop(worker->task);
> +	put_task_struct(worker->task);
>  	kfree(worker);
>  
>  	spin_lock_irq(&pool->lock);